Oscillation control of structures under dynamic loads such as earthquake, wind, ground motion and vibration caused by vehicles and machineries movement attract huge interest among structural engineers and researchers. Seismic vibration can cause excessive oscillations of the building which may lead to structural catastrophic failure. Improvement of seismic performance in terms of safety is one of the most concerns in seismic design of structures. Therefore, proper building design and vibration control technologies are implemented to avoid the destructive building failure.
In the last two decades, a lot of research has been done to enhance seismic resistance structural system and control technique to achieve the more economical and safer design (Spencer and Nagarajaiah, 2003). As mentioned above, the traditional seismic design philosophy contains the dissipation of input seismic energy by aid of inherent ductility capacity of structural element through large strains in aforementioned components. In contrary, this approach may lead to structural damage or unrealistic design. For this reason, utilize of energy dissipation devices which is not belonging to the main load resisting system was suggested and designed specifically as external devices for absorption of seismic energy. These devices can be simply substituted after severe excitation (Soong and Dargush, 1997; Symans et al., 2008).
A variety of control schemes have been employed in design practices and generally can be categorized into three types: active control (Yao, 1972), passive control and semi active control (Crosby et al., 1974) Among these methods, passive control systems were developed at the earliest phase, and have been utilized more frequently and practically in seismic design procedure due to the minimum maintenance necessitate and eliminate the external power supply to function. In high seismicity regions, steel moment resisting frames (MRSF) are regularly selected due to adequate energy dissipation capacity, which is granted by large plastic deformation of elements in the moment frames (Bruneau, 1998). This ability permits the structural engineers to design the moment resisting frames under the lowest lateral design force compared with other structural systems. Nevertheless, unanticipated severe events might bring unacceptable great storey displacement. Prior vigorous earthquake events have emphasized the need of seismic retrofitting of present moment frames.
In the recent years, active variable stiffness (AVS), a system for structural control has absorbed numerous attentions and interests. The desire effects and improvement of the structural performance in earthquake excitation of AVS systems were proven by previous studies (Kobori, 1993; Yang et al., 1996). Such a system has been investigated experimentally with implementation in full-scale building in Japan (Kamagata and Kobori, 1994; Kobori and Kamagata, 1992). Most of available variable stiffness system are operated using external electrical controller which may cause delay in system performance. These systems are highly depended on energy recourse and also need repetitive maintenance.
One of the examples of such devices is found in U.S. Pat. No. 6,923,299 where a variable spring member includes a containment housing defining an inner chamber with alternating layers of compressible medium and electro-reactive medium. Adjacent each layer of electro-reactive medium is a coil assembly controlled by a controller. A sealed plate disposed between alternating layers of compressible medium and electro-reactive medium disperses a load exerted on the variable spring member assembly and prevents intermixing of compressible medium with the electro-reactive medium. Actuation of the coil assembly changes physical characteristics and compressibility of the layer of electro-reactive medium to vary spring rate and stiffness.
Therefore it is required to invent a real time system/device which independent of the energy recourse and maintenance procedure.